If you are using the external display that’s made by the apple.Then, you’ll find a Brightness slider in the system preference. Now, you can control Mac’s brightness by moving the slider from left to right. You can also click on Automatically adjust brightness.

Note: This method won’t work. If you are using an external display which is not made by Apple.

If you are using another external display or any third party display. Then, you can find the buttons on the monitor itself. You can find the key or buttons on the external display. You can also get the option of brightness from your Menu tab. You need to manually press these buttons to adjust your Mac’s Screen brightness.

Note: If you uncheck. Then, it will increase the brightness and consume your battery faster.

Automatically Adjust Brightness based on the room light level or automatic brightness

Mac consists of several sensors. These sensors help in detecting various thing like Temperature, Light etc. The ambient light sensor is one of the most important features of Mac. It helps in adjusting Mac brightness when you are outdoors or indoors. If you are using Mac outside and lots of sunlight coming. Then, it will become hard for your to view. At that time, Mac automatically increases the brightness and increase the experience of using Mac. In a similar method, it dims the light when you are in dark or at night. It also helps in saving your battery.